1. Field of the Invention
The present invention relates to a semiconductor device such as a thin film transistor (hereinafter referred to as TFT) which is formed on a substrate using a crystalline semiconductor film as well as to a manufacturing method of the same. The semiconductor device of the present invention includes not only elements such as TFT and MOS transistor, but also a liquid crystal display device comprising a semiconductor circuit (micro-processor, signal processing circuit and high frequency circuit and the like) constructed of the above insulation gate type transistors, an EL (Electro Luminescence) display device, an EC (Electro Chromic) display device and an image sensor and the like.
2. Description of the Related Art
At present, as a semiconductor element using a semiconductor film, a thin film transistor (hereinafter, referred to as TFT) is used in a variety of integrated circuits, in particular, as a switching element of an image display device. In addition, a TFT using a crystalline semiconductor film, of which the mobility is higher than that of an amorphous semiconductor film, as an active layer has a high driving performance and is used as an element of a driving circuit.
Reliability is seen as the most important factor in evaluating a TFT. As for a factor that decreases reliability, impurities in a TFT (hereinafter, impurities which lower the reliability of a TFT are referred to as contaminant impurities in the present specification) are cited. These contaminant impurities become mixed into a TFT from a variety of contamination sources, such as an atmosphere, a glass substrate or a manufacturing device, etc. In particular, the existence of contaminant impurities on the interfaces of a film forming a TFT becomes a major factor that decreases the reliability of the TFT.
A semiconductor device is usually produced in a clean room. In a clean room, filters are used for removing dirt, dust, contaminants, or the like from the outside air which is taken into the room. Contaminant impurities generated from the filters themselves, particularly boron (B), or contaminant impurities from human beings working in the clean room, particularly sodium (Na), exist in large amounts in the clean room atmosphere. That is to say, by merely exposing a substrate to the atmosphere of the clean room, the surface of the substrate gets contaminated.
Most of the steps of forming a variety of films which form a TFT, however, cannot be carried out in a sequential process. For example, a crystalline semiconductor film is obtained by using a method which crystallizes an amorphous semiconductor film with heat, laser light, or the like. Crystallization of the amorphous semiconductor film with annealing becomes difficult in the case that an insulating film lies on the amorphous semiconductor film and, therefore, the crystallization is carried out under the condition where no insulating film exists on the semiconductor film. Accordingly, it is necessary to wash the surface of the semiconductor film before the formation of the insulating film. That is to say, the step of washing the surface of the first film before the formation of the second film becomes necessary. During the time before being put into a film formation unit from a washer, however, a substrate is exposed to the atmosphere.
The present invention is provided for maintaining the clean condition of an interface of each film forming a TFT without being contaminated by contaminant impurities.
In order to solve the above described problem, the present invention provides a washing chamber for removing contaminant impurities from the surface of the first film with the film formation unit via a loader/unloader chamber, and is characterized by the sequential processing from the removal of contaminant impurities from the surface of the first film, to the formation of the second film. In other words, the present invention is characterized in that, since the step of washing the surface of the first film and the step of forming the second film are processed in sequence within one unit, the above steps are processed without exposure to the atmosphere including the interval between them.
In the above described configuration, the removal of contaminant impurities from the surface of the first film is carried out by etching the surface of the first film extremely thin using an acid solution which includes fluorine, after carrying out the washing with pure water in which ozone is dissolved. As for the means of etching extremely thin, there is an effective method of spinning the substrate using a spinning device so as to make the acid solution, which includes fluorine and is in contact with the surface of the first film, fly off in the air in all directions.
The reasons why the above pure water, in which ozone is dissolved, is used are cited as follows:
(1) An extremely thin oxide film is formed on the surface of the first film by means of ozone so that contaminant impurities adhered to the surface of said first film can be removed together with said extremely thin oxide film by using the subsequent acid solution which includes fluorine.
(2) In the case that the first film is hydrophobic, by oxidizing the surface thereof with ozone, the surface of the first film is converted to be hydrophilic so that the effects of washing increase.
(3) A microscopic amount of a carbonized substance which may exist in the clean room atmosphere can be removed through ozone oxidation and the resulting resolution.
As for the acid solution which includes fluorine, fluoric acid, dilute fluoric acid, ammonium fluoride, buffered fluoric acid (mixed solution of fluoric acid and ammonium fluoride), a mixed solution of fluoric acid and hydrogen peroxide, or the like, can be used.
Accordingly, a semiconductor device of the present invention is characterized in that contaminant impurities of the surface of each film forming a TFT are removed by providing a spinning type washer in the washing chamber and by using pure water in which ozone is dissolved, and an acid solution containing fluorine, as a washing liquid.
In accordance with the present invention, contamination, due to contact with air, of the interfaces of the films which form TFTs can be prevented and, therefore, dispersion of the characteristics of the TFTs can be reduced so that the reliability of the TFTs can be increased. In addition, the instability of the display of an EL display device, which is greatly affected by the dispersion of the characteristics of the TFTs because of current drive, can be reduced.